1. Field of the Invention
The present invention relates to an annual date mechanism for a timepiece movement comprising a 31-toothed date runner, a jumper in mesh with its toothset, a months satellite, the rotation pin of which is secured to this date runner and which comprises five driving teeth of a toothset on a pitch for twelve for the months comprising less than 31 days, a fixed planetary toothset coaxial with the date runner and in a direct-drive relationship with the months satellite and a drive member for driving the date runner in a driving relationship with the hours wheel of the timepiece movement and comprising two drive fingers, the first intersecting the path of the toothset of the date runner, the second intersecting the path of the toothset of the months satellite when its axis of revolution is aligned with those of the planetary toothset, of the drive member and of the date runner.
2. Description of Related Art
Such an annual date mechanism, associated with a perpetual calendar mechanism, is described in EP 1 351 104. This mechanism comprises a months satellite the pivot pin of which is secured to a date wheel which makes one revolution per month. This months satellite has twelve teeth, seven of which are truncated and five of which are not. The twelve teeth of this satellite mesh with a fixed 7-tooth planetary toothset coaxial with the date wheel.
During the year, for each revolution of the date wheel, the toothset of the months satellite occupies a different position when its axis of pivoting is aligned with the axis of the planetary toothset and the axis of pivoting of a wheel which makes one revolution every twenty-four hours in order to drive the date wheel. For this purpose, this twenty-four hour wheel has twenty-four teeth, twenty of which are truncated and of the other four, one is a normal drive tooth which meshes with the date wheel once per day and another is an annual correction tooth, offset parallel with its axis of rotation, to come into mesh with one of the five un-truncated teeth of the months satellite each time the month contains less than thirty-one days.
When the month comprises less than 31 days, one of the five un-truncated teeth of the months satellite covers one of the teeth of the data wheel and is situated in the path of a correcting tooth of the wheel which makes one revolution in twenty-four hours so that by turning, the correction tooth of this wheel, offset parallel to its axis of rotation, causes the months satellite to turn, which satellite, being in mesh with the fixed planetary toothset, causes the date wheel to turn before the normal driving finger driving this twenty-four hour wheel causes the date wheel to turn by one step, as it does on each rotation, so that the date wheel is moved by two steps for one revolution of the twenty-four-hour wheel.
This mechanism has the advantage of avoiding the cams and lever devices like those described in CH 685 585 or in EP 987 609, which use energy, are tricky to develop and are therefore not very reliable.
Although the design is tempting, this mechanism does, however, exhibit a substantial disadvantage stemming from the fact that the months satellite works on a first pitch circle with the fixed planetary toothset, whereas it works on a second pitch circle, larger than the first, with the drive teeth of the twenty-four hour wheel. This larger pitch diameter is needed to prevent the drive teeth of the twenty-four hour wheel from being able to mesh with the truncated teeth of the months satellite. In consequence, the penetration between the teeth of the twenty-four hour wheel in the toothset of the months satellite is shallow, and the magnitude of the drive angle is small. Such a mechanism is not therefore very reliable and at the very least is extremely difficult to optimize, leading to item by item readjustment.
An additional disadvantage with this solution stems from the fact that when the axis of revolution of the months satellite is aligned with the respective axes of revolution of the planetary toothset and of the twenty-four hour wheel, the satellite lies between them, which means that this satellite is driven on part of its toothset situated furthest from the center of the date wheel by the twenty-four-hour wheel situated on the outside of this date wheel, reducing the drive angle to a minimum, the penetration and the drive angle already being small because the pitch diameter between this twenty-four-hour wheel and the months satellite is enlarged with respect to the pitch diameter between this satellite and the planetary toothset. Production and development of such a mechanism is therefore problematic and its reliability is poor.
It can therefore be concluded from this that, in spite of there being a solution which is the subject of EP 1 351 104, no credible alternative to the current date mechanisms that employ cams and levers has yet been proposed.